The reaction conditions for the catalytic reduction of single-ring aromatic compounds such as tetralin with chloro(1,5-hexadiene)rhodium(I) dimer in a biphasic mixture of hexane and an aqueous buffer containing a surface active agent have been investigated. The reaction proceeds most effectively in a dilute alkaline buffer with a low concentration of a surfactant. Under these conditions, the hydrogenation of tetralin provided cis-and trans-decalin in high yield with no apparent decrease in the catalytic activity after eight catalyst cycles. The reaction rate increases as the pressure and the temperature are increased. The reaction system is effective for the reduction of tetralin in the presence of coal liquids.